22 September 2017

TE-204 (AN/USC-11) is most commonly used by Allied Air Forces as an air-to-ground messaging system as well as in ground and naval applications. The transmission was heard on 11220.0 KHz/USB, HF-GCS frequency, at 2145 UTC.The modem converts the incoming serial binary data stream into an FSK audio signal at baseband and appears on-air as MFSK-4 150Bd/440Hz (Figure 1) although the real transfer speed is 75 Baud (75 bit/s in synchronous mode, 75 baud using Baudot chars in the async mode): the reason is in the "in-band frequency and time diversity" mode used by this modem.

Fig. 1 Over-the-air parameters

Four data subcarriers are used at 935 Hz through 2255 Hz with tones spaced every 440 Hz: TE-204 transmits a "mark" on 935 Hz for 6.67 msec period followed by another 6.67 msec period at 1815 Hz. Similarly, a "space" is transmitted at 1375 Hz for 6.67 msec period followed by another 6.67 msec period at 2255 Hz (Fig. 2).

Fig. 2 in-band frequency and time diversity mode

This effectively provides an in-band frequency diversity function for each data bit while also transmitting the same data bit over two(!) separate 6.67 msec periods of time, thus achieving a in-band frequency and time diversity function while transmitting only one tone at time (so the speed is the half of the measured one). As for above, from the perspective of the data-transfer, the modem works as a FSK-2 75Bd/880Hz modem.

No special preamble or SOM/EOM codes are employed, decoding the signal as in the depicted mode will be possible to get the 64-bit KG-84 sync sequence.

By the way, the TE-204 modem is embedded in the Rockwell Collins MDM2001 Multimode HF Modem.